The invention relates to a process for cleaning a cold trap incorporated into a liquid metal circuit. This type of trap is most frequently used in a sodium or sodium-potassium alloy circuit and serves to carry out the crystallisation and elimination of impurities such as, in the case of a sodium circuit, sodium hydride and sodium oxide. The cleaning of the trap makes its reuse for storage possible, preferably after decontamination. The invention also relates to an apparatus for performing this process.
In high and medium power nuclear reactors, it is necessary to have a cooling fluid, whose thermal conductivity coefficient is high, in such a way as to be able to utilise to the maximum the power possibilities of the reactor. For this purpose and despite the difficulties resulting from the use of such materials, liquid metals are frequently used for extracting the heat supplied by such reactors. Sodium is one of the metals used in this way. Due to the very special circulation conditions of the liquid metal for cooling the reactor, impurities are formed in this metal which are eliminated by means of at least one cold trap arranged in the circuit. The main impurities trapped in this way are, in the case of sodium, sodium hydride (NaH), sodium oxide (Na.sub.2 O) and sodium hydroxide (NaOH). Due to the temperature of the trap, these impurities are in the solid state in such a way that after the reactor has been in use for a certain time it may become necessary to clean the trap or traps located in the circuit, due to the reduction in the purified sodium flow and even the danger of clogging. Various processes have already been proposed for cleaning a cold trap incorporated into a sodium circuit.
According to a first known process, the trap is heated at about 420.degree. C. and is scavenged with neutral gas. Under these conditions, the sodium hydride decomposes in accordance with the following reaction (1): EQU NaH (s)+L.sub.1 +1/2 H.sub.2 (g) (1)
in which L.sub.1 is a liquid solution of hydrogen and oxygen in the sodium, the oxygen coming from the sodium oxide which is still present in the cold trap at the same time as the hydride.
Thus, this process permits the total decomposition of the sodium hydride. However, only a small fraction of the sodium oxide is converted into hydroxide according to reaction (2): EQU Na.sub.2 (s)+1/2 H.sub.2 (g).fwdarw.L.sub.1 +L.sub.2 ( 2)
in which L.sub.2 is a liquid solution of hydrogen and oxygen in sodium hydroxide.
Thus, the slow kinetics of the latter reaction do not permit its completion during the cleaning period. Thus, this process is not suitable because it does not permit the total transformation of the sodium oxide within reasonable periods of time. Moreover, the tritium present in the cold trap is discharged in the gaseous effluents, which is a disadvantage.
According to a cleaning process, thus described in French Patent No. 2,312,276, the sodium hydride and sodium oxide are transformed into sodium hydroxide by mutual reaction beyond 410.degree. C. The reaction which occurs is as follows: EQU NaH (s)+Na.sub.2 O (s).fwdarw.L.sub.1 +L.sub.2 ( 3)
If the sodium hydride is in excess compared with the sodium oxide in the trap, the hydride is eliminated by decomposition as in the process described hereinbefore according to reaction (1). However, if the sodium oxide is in excess, it is transformed into the sodium hydroxide by hydrogenation according to reaction (2).
This process has the advantage of being more complete than the previous process, because it permits the transformation of both sodium hydride and sodium oxide, but also has the disadvantage, like the previous process, of having to be performed at a temperature above 410.degree. C. and of being slow.